A long time ago we used to track the dirty states at the PG/NG layer and so the NG nodes listen to changes in the Decora effect classes so they can mark their nodes dirty when the effects change.

Since then we've moved almost all dirty processing up to the FX layer and the FX layer even creates weak listeners on the FX-level effect classes. As a result the listeners used at the NG layer are redundant.

Unfortunately, as was discovered in trying to eliminate those lower level listeners in trying to fix RT-15517, the FX-level effect change propagation is incomplete so we won't redraw the tree properly if we don't add the listeners at the NG level as well. We should eventually remove those lower level listeners and finish the implementation of he FX layer effect dirty state propagation.